The present invention relates to recording materials suitable for thermographic or photothermographic development. In particular, it concerns materials with improved shelf-life and prints with improved archivability produced with such materials.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In direct thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes color or optical density. Such recording materials become photothermographic upon incorporating a photosensitive agent, which after exposure to UV, visible or IR light, is capable of catalyzing or participating in a thermographic process bringing about changes in color or optical density.
Most of the xe2x80x9cdirectxe2x80x9d thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a colored image is produced.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive (thermographic) copy paper includes in the heat-sensitive layer a thermoplastic binder, a water-insoluble silver salt and an appropriate organic reducing agent. Thermosensitive copying paper is used in xe2x80x9cfront-printingxe2x80x9d or xe2x80x9cback-printingxe2x80x9d using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
GB-A 1,542,327 discloses a thermally developable light-sensitive sheet material, comprising a support and (a) a silver salt of an organic acid, (b) a catalyst in an amount capable of catalyzing the reaction in exposed areas of the material of components (a) and (c) after imagewise exposure and heating of the material, (c) a reducing agent for the silver salt, (a), and (d) sulphur in an amount to reduce the color change after processing and to reduce thermal fogging, the components (a) to (d) are contained, separately or together, in one or more layers coated on the support or being all present in the support or one or more of the components being present in the support and the remainder being in one or more layers coated thereon. The description of GB 1,542,327 includes an exhaustive list of silver salts of organic acids including silver palmitate. Furthermore, D. S. Avose, V. V. Tsvetkov and V. D. Yagodovskii in Sci. Appl. Photo. volume 35, pages 587-594 published in 1994 by Gordon and Breach Science Publishers S.A. describe photothermographic materials based on silver bromide and silver palmitate.
U.S. Pat. No. 4,273,723 discloses a process for preparing a silver salt of a fatty acid with 12 to 24 carbon atoms consisting essentially of reacting an alkali metal salt of the fatty acid with a water-soluble silver salt. The reaction is effected in a reaction system consisting essentially of (I) the alkali metal salt of the fatty acid, (II) the water-soluble silver salt, (IR) at least one water-soluble or partially water-soluble C3-C8 alcohol and (IV) water, the volume ratio of the component (III) to the component (IV) being from 1/5 to 5/1.
EP-A 754,969 discloses a process for producing a suspension of particles containing a substantially light-insensitive silver salt of an organic carboxylic acid. The process comprises simultaneous metered addition of an aqueous solution or suspension of an organic carboxylic acid or its salt and an aqueous solution of a silver salt to an aqueous liquid. The metered addition of the aqueous solution or suspension of the organic carboxylic acid or its salt and/or the aqueous solution of the silver salt is regulated by the concentration of silver ions or the concentration of anions of the silver salt in the aqueous liquid.
Research Disclosure number 17029, published in June 1978, in section II gives a survey of different methods of preparing organic heavy metal salts. Method 5, for example, describes the preparation of silver behenate by (a) heating behenic acid in water to a temperature above the melting point of the acid, but below the boiling point of the dispersion, (b) adding an aqueous solution of alkali metal or ammonium hydroxide, and (c) adding an aqueous solution of silver nitrate. However, in order to obtain a fine emulsion of an organic heavy metal salt, either the synthesis has to be carried out in an organic solvent medium as disclosed, for example, in U.S. Pat. No. 3,700,458 or in a mixture of water and a substantially water insoluble organic solvent as disclosed, for example, in U.S. Pat. No. 3,960,908 for silver carboxylates.
GB-A 1,378,734 discloses a process of producing a silver salt of an organic carboxylic acid, which comprises mixing (a) an aqueous solution of silver nitrate or a silver complex with (b) a solution of an organic carboxylic acid in a solvent in which the organic carboxylic acid is soluble. The solvent is chosen so that both the silver salt of an organic carboxylic acid and the silver nitrate are almost insoluble in the solvent and water is sparingly miscible in the solvent. In the mixture, the organic carboxylic acid reacts with silver ions. The reaction is conducted in the presence of a soluble mercury compound and/or a soluble lead compound.
The association of silver behenate, silver palmitate, or silver stearate with mercury or lead ions, particularly mercury ions, as disclosed in GB 1,378,734 is environmentally undesirable and infringes governmental regulations. Direct thermal recording materials with silver behenates produced using the processes described in RD 17029 exhibit a brown image color upon image-wise heating which is undesirable for medical images viewed in transmission with a viewing box. This lack of image color neutrality can be quantified by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90 to produce the CIELAB a* and b* co-ordinates. Color neutrality on the basis of CIELAB-values corresponds to a* and b* values of zero, with a negative a*-value indicating a greenish image-tone becoming greener as a* becomes more negative, a positive a*-value indicating a reddish image-tone becoming redder as a* becomes more positive, a negative b*-value indicating a bluish image-tone becoming bluer as b* becomes more negative and a positive b*-value indicating a yellowish image-tone becoming yellower as b* becomes more positive.
Additionally, recording materials prepared using prior art silver palmitate processes and compounds exhibit poor shelf-life. Recording materials prepared using prior art silver stearate processes and compounds also exhibit poor shelf-life and prints produced with these materials exhibit poor archivability, particularly as regards increase in Dmax.
It is therefore an object of the invention to provide recording materials with improved shelf-life.
It is a further object of the present invention to provide thermographic and photothermographic recording materials whose prints exhibit improved archivability and light stability.
It is a still further object of the present invention to provide a production process for substantially light-insensitive organic silver salts comprising silver behenate, palmitate, and/or stearate.
Further objects and advantages of the invention will become apparent from the description below.
Surprisingly, it has been found that recording materials comprising a support and a thermosensitive element comprising silver behenate, silver palmitate or silver stearate with a higher crystallinity than respective prior art organic silver salts, an organic reducing agent therefor in thermal working relationship therewith and a binder exhibit a marked improvement in shelf-life and/or archivability of prints produced with the recording materials relative to recording materials of the prior art.
The above mentioned objects of the invention are realized with a recording material comprising a support and a thermosensitive element containing silver behenate, silver palmitate or silver stearate, an organic reducing agent therefor in thermal working relationship therewith, and a binder. In the present invention the silver behenate, silver palmitate, and silver stearate are not associated with mercury and/or lead ions. When the recording material comprises silver behenate and is irradiated with a copper Kxcex11 X-ray source, the ratio, normalized to a quantity of silver in the recording material of 1 g per m thereof, of the sum of the peak heights of the X-ray diffraction lines attributable to silver behenate at Bragg angles, 2"THgr" of 6.01xc2x0, 7.56xc2x0, 9.12xc2x0, 10.66xc2x0, 12.12xc2x0 and 13.62xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.6xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST standard 1976, rhombohedral Al2O3, determined wit the same X-ray diffractometer in the same state of adjustment on a sample of the recording material and a sample of the NIST standard 1976 cut to fit a sample holder of the X-ray diffractometer, is greater than 0.85 m2/g.
When the recording material comprises silver palmitate and is irradiated with a copper Kxcex11 X-ray source, the ratio of the sum of the peak heights of the X-ray diffraction lines attributable to silver palmitate at Bragg angles, 2"THgr", of 4.01xc2x0, 6,049xc2x0, 8,031xc2x0, 10.06xc2x0, 12.08xc2x0 and 14.09xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST standard 1976, rhombohedral Al2O3, determined with the same X-ray diffractometer in the same state of adjustment on a sample of the recording material and a sample of the NIST standard 1976 cut to fit a sample holder of the X-ray diffractometer, divided by the square root of the quantity of silver in the recording material, expressed in g per m2, is greater than 3.09 m/g0.5.
When the recording material comprises silver stearate and is irradiated with a copper Kxcex11 X-ray source, the ratio of the sum of the peak heights of the X-ray diffraction lines attributable to silver stearate at Bragg angles, 2"THgr", of 3.62xc2x0, 5.45xc2x0, 7.30xc2x0, 9,04xc2x0, 10.97xc2x0 and 12.71xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST standard 1976, rhombohedral Al2O3, determined with the same X-ray diffractometer in the same state of adjustment on a sample of the recording material and a sample of the NIST standard 1976 cut to fit a sample holder of the X-ray diffractometer, divided by the square root of the quantity of silver in the recording material, expressed in g per m2, is greater than 2.2 m/g0.5.
A recording process according to the present invention is also provided comprising: (i) bringing an outermost layer of an above described recording material in proximity with a heat source; and (ii) applying heat from the source to the recording material while maintaining proximity to the heat source to produce an image; and (iii) removing the recording material from the heat source.
A production process for particles of substantially light-insensitive organic silver salts comprising silver behenate according to the present invention is also provided comprising: i) producing a solution or dispersion, A, comprising an alkali metal or ammonium salt of an organic compound with at least one acidic hydrogen atom in a mixture of water and an organic solvent at a temperature at which particles of the substantially light-insensitive organic silver salt comprising silver behenate do not undergo reduction; and ii) adding a quantity of an aqueous solution, B, of a silver salt containing an equal number of silver ions to the alkali or ammonium ions in the solution or dispersion A. The process is further characterized in that the initial mixing number during the addition of the aqueous solution B to the solution or dispersion A is greater than or equal to 2xc3x9710xe2x88x924 during the production of the particles of substantially light-insensitive organic silver salt. The mixing number is the ratio of the molar rate at which the silver salt in solution B is supplied to solution A in a reactor to the molar rate at which the alkali or ammonium salt is circulated in the reactor.
A production process for a dispersion of particles of substantially light-insensitive organic silver salts primarily including a silver salt of an organic carboxylic acid in a substantially solvent-free aqueous medium according to the present invention is also provided. The process comprises: i) preparing an aqueous dispersion of one or more organic acids primarily including the organic carboxylic acid and an anionic surfactant; ii) substantially neutralizing the organic acids with aqueous alkali, thereby forming organic acid salts primarily including the salt of the organic carboxylic acid; (iii) adding an aqueous solution of a silver salt to completely convert the organic acid salt(s) into their silver salts primarily including the silver salt of the organic carboxylic acid. During the production process, the anionic surfactant is present in a molar ratio with respect to organic acid greater than 0.15 and the silver salt is added at a rate between 0.025 mol/(mol organic silver saltxc3x97min) and 2.25 mol/(mol organic silver saltxc3x97min).
Preferred embodiments of the invention are disclosed in the detailed description.
In a preferred embodiment of the recording process of the present invention, the heat source is a thermal head. In a more preferred embodiment, it is a thin film thermal head.
Substantially light-insensitive, as used in this application, means not intentionally light sensitive. A substantially solvent-free aqueous medium as used in this application is a medium in which a solvent, if present, is present in amounts below 10% by volume of the aqueous medium.
The silver behenate, silver palmitate and silver stearate of the present invention is not associated with mercury and/or lead ions. This means that mercury/and or lead ions are not intentionally added at any point during the preparation process and, therefore, are not intentionally associated with the silver behenate, silver palmitate and silver stearate present in the recording material of the present invention.
The thermosensitive element, according to the present invention, comprises one or more of silver behenate, silver palmitate and silver stearate, an organic reducing agent therefor in thermal working relationship therewith, and a binder. The element may comprise a layer system in which the ingredients may be dispersed in different layers, provided that the two ingredients are in reactive association with one another. However, during the thermal development process, the reducing agent must be present in such a way that it is able to diffuse to the silver behenate, silver palmitate and silver stearate present so that reduction of the silver behenate, silver palmitate and silver stearate present to silver can occur, giving the desired image-tone.
In a preferred embodiment of the present invention the thermosensitive element further comprises a photosensitive species capable, upon exposure to light, of forming another species capable of catalyzing reduction of the silver behenate, silver palmitate, and silver stearate present.
If the organic silver salt in the recording material of the present invention primarily comprises silver behenate then, when irradiated with a copper Kxcex11 X-ray source the ratio, normalized to a quantity of silver in the recording material of 1 g per m2 thereof, of the sum of the peak heights of the X-ray diffraction lines attributable to silver behenate at Bragg angles, 2"THgr", of 6.01xc2x0, 7.56xc2x0, 9.12xc2x0, 10.66xc2x0, 12.12xc2x0 and 13.62xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST (National Institute of Standards, Gaithersburg, Md. 20899-0001, USA) standard 1976, rhombohedral Al2O3, determined with the same X-ray diffractometer in the same state of adjustment, is greater than 0.85 m2/g. In a preferred embodiment of the present invention, the normalized ratio, as defined above, is greater than 1.0 m2/g and in a particularly preferred embodiment is greater than 1.2 m2/g.
The normalized ratio is obtained by determining X-ray diffraction spectra on sheets of a particular recording material and of the NIST standard 1976 cut to fit the sample holder of the X-ray diffractometer used, subtracting the background using standard techniques, determining the peak heights of the diffraction peaks, determining for the sample of recording material the sum of the peak heights, Kmaterial, of the XRD lines attributable to silver behenate at Bragg angles, 2"THgr", of 6.01xc2x0, 7.56xc2x0, 9.12xc2x0, 10.66xc2x0, 12.12xc2x0 and 13.62xc2x0, determining for the sample of NIST standard 1976 the sum of the peak heights, K1976, of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0, calculating the ratio of Kmaterial/K1976 for the recording material, determining the concentration of silver CAg present in the recording material in grams per square meter of material and finally normalizing the ratio Kmaterial/K1976 with CAg to give the normalized ratio Kmaterial/(K1976xc3x97CAg), which is referred to in the detailed description of the present invention as the crystallinity of silver behenate. The exact positions of the peaks attributable to silver behenate can vary within 0.3xc2x0 of the angles given above. In such cases the peak height should be taken as the actual peak height of the peak and not the height of the peak at the angle given above.
The concentration of silver present in the recording material can be determined by any known technique e.g. non-destructive methods such as X-ray fluorescence and destructive methods such as dissolution of the silver salt followed by standard volumetric techniques for the determination of silver, such as described in R. Belcher and A. J. Nutten, Quantitative Inorganic Analysis, 2nd Edition, Butterworths, London (1960), pages 201-219.
A standard test was used to assess the image tone of thermographic recording materials comprising silver behenate of the present invention. This consisted of first coating a subbed 175 xcexcm thick polyethylene terephthalate support with a solvent dispersion comprising: silver behenate (AgB), 400% by weight relative to AgB of polyvinylbutyral, 50 mol % relative to AgB of ethyl 3,4-dihydroxybenzoate, 15 mol % relative to AgB of benzo[e][1,3]oxazine-2,4-dione, 5 mol % relative to AgB of 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione, 0.9 wt % of silicone oil relative to AgB, 5 mol % relative to AgB of tetrachlorophthalic anhydride, 21.98 mol % relative to AgB of adipic acid and 10 mol % relative to AgB of benzotriazole to a coating weight of AgB of about 5 g/m2. After drying for 1 hour at 50xc2x0 C., the thermographic recording material was tempered for 7 days at 45xc2x0 C. Thermal printing was carried out with the print head separated from the thermosensitive layer by a separable 5 xcexcm thick polyethylene terephthalate ribbon coated successively with a subbing layer, heat-resistant layer and a slipping layer (anti-friction layer) and a printer equipped with a thin film thermal head with a resolution of 300 dpi and operated with a line time of 19 ms (the line time being the time needed for printing one line). During the line time the print head received constant power. The average printing power, being the total amount of electrical input energy during one line time divided by the line time and by the surface area of the heat-generating resistors was 1.5 mJ/dot. The image tone was assessed both by visual inspection and on the basis of the L*, a* and b* CIELAB-values of the image as a function of the optical density of the image determined with a MacBeth(trademark) TR924 densitometer. The L*, a* and b* CIELAB-values were determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90. The value of b* at the minimum in the dependence of b* upon image density of less than xe2x88x922.0 was found to correspond with an image with a blue tone compared with images with a brown tone for minima with higher b* values.
If the organic silver salt in the recording material of the present invention primarily comprises silver palmitate, when the recording material is irradiated with a copper Kxcex11 X-ray source the ratio of the sum of the peak heights of the X-ray diffraction lines attributable to silver palmitate at Bragg angles, 2"THgr", of 4.01xc2x0, 6,049xc2x0, 8,031xc2x0, 10.06xc2x0, 12.08xc2x0 and 14.09xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST (National Institute of Standards, Gaithersburg, Md. 20899-0001, USA) standard 1976, rhombohedral Al2O3, determined with the same X-ray diffractometer in the same state of adjustment, divided by the square root of the quantity of silver in the recording material, expressed in g per m2, is greater than 3.09 m/g0.5, which is referred to in the detailed description of the present invention as the crystallinity of silver palmitate. In a preferred embodiment of the present invention, the crystallinity of silver palmitate is greater than 3.3 m/g0.5 and, in a particularly preferred embodiment, is greater than 3.8 m/g0.5.
The crystallinity of the silver palmitate in the recording material of the present invention is obtained by determining X-ray diffraction spectra on sheets of a particular recording material and of the NIST standard 1976 cut to fit the sample holder of the X-ray diffractometer used, subtracting the background using standard techniques, determining the peak heights (maxima) of the diffraction peaks, determining for the sample of recording material the sum of the peak heights (maxima), Kmaterial, of the XRD lines attributable to silver palmitate at Bragg angles, 2"THgr", of 4.01xc2x0, 6,049xc2x0, 8,031xc2x0, 10.06xc2x0, 12.08xc2x0 and 14.09xc2x0, determining for the sample of NIST standard 1976 the sum of the peak heights (maxima), K1976, of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0, calculating the ratio of Kmaterial/K1976 for the recording material, determining the concentration of silver CAg present in the recording material in grams per square meter of material and finally normalizing the ratio Kmaterial/K1976 with CAg to give Kmaterial/(K1976 xc3x97CAg), which is a relative crystallinity for the silver palmitate in the recording material concerned. The exact positions of the peaks attributable to silver palmitate can vary within 0.3xc2x0 of the angles given above. In such cases the peak height should be taken as the actual peak height of the peak and not the height of the peak at the angle given above.
The concentration of silver present in the recording material can be determined by any known technique, e.g. non-destructive methods such as X-ray fluorescence and destructive methods such as dissolution of the silver salt followed by standard volumetric techniques for the determination of silver, as described in R. Belcher and A. J. Nutten, Quantitative Inorganic Analysis, 2nd Edition, Butterworths, London (1960), pages 201-219.
If the organic silver salt in the recording material of the present invention primarily comprises silver stearate when the recording material is irradiated with a copper Kxcex11 X-ray source the ratio of the sum of the peak heights of the X-ray diffraction lines attributable to silver stearate at Bragg angles, 2"THgr", of 3,62xc2x0, 5.45xc2x0, 7.30xc2x0, 9,04xc2x0, 10.97xc2x0 and 12.71xc2x0 to the sum of the peak heights of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0 of NIST (National Institute of Standards, Gaithersburg, Md. 20899-0001, USA) standard 1976, rhombohedral Al2O3, determined with the same X-ray diffractometer in the same state of adjustment on a sample of the recording material and a sample of the NIST standard 1976 cut to fit a sample holder of the X-ray diffractometer, divided by the square root of the quantity of silver in the recording material, expressed in g per m2, is greater than 2.2 m/g0.5, which is referred to in the detailed description of the present invention as the crystallinity of silver stearate. In a preferred embodiment of the present invention, the crystallinity of silver stearate is greater than 3.0 m/g0.5.
The crystallinity of the silver stearate in the recording material of the present invention is obtained by determining X-ray diffraction spectra on sheets of a particular recording material and of the NIST standard 1976 cut to fit the sample holder of the X-ray diffractometer used, subtracting the background using standard techniques, determining the peak heights (maxima) of the diffraction peaks, determining for the sample of recording material the sum of the peak heights (maxima), Kmaterial, of the XRD lines attributable to silver stearate at Bragg angles, 2"THgr", of 3.62xc2x0, 5.45xc2x0, 7.30xc2x0, 9.04xc2x0, 10.97xc2x0 and 12.71xc2x0, determining for the sample of NIST standard the sum of the peak heights (maxima), K1976, of the X-ray diffraction lines at Bragg angles, 2"THgr", of 25.60xc2x0, 35.16xc2x0 and 43.40xc2x0, calculating the ratio Of Kmaterial/K976 for the recording material, determining the concentration of silver CAg present in the recording material in grams per square meter of material and finally normalizing the ratio Kmaterial/K1976 with CAg to give Kmaterial/(K1976 xc3x97CAg), which is a relative crystallinity for the silver stearate in the recording material concerned. The exact positions of the peaks attributable to silver stearate can vary within 0.3xc2x0 of the angles given above. In such cases the peak height should be taken as the actual peak height of the peak and not the height of the peak at the angle given above.
The concentration of silver present in the recording material can be determined by any known technique e.g. non-destructive methods such as X-ray fluorescence and destructive methods such as dissolution of the silver salt followed by standard volumetric techniques for the determination of silver, such as described in R. Belcher and A. J. Nutten, Quantitative Inorganic Analysis, 2nd Edition, Butterworths, London (1960), pages 201-219.
By the term xe2x80x9cprimarily includes a silver salt of an organic carboxylic acidxe2x80x9d it is meant that more of the particular silver salt of an organic carboxylic acid is present in the particle than other types of organic silver salts e.g. in the case of silver behenate than other organic silver salts such as silver palmitate and silver stearate. The term xe2x80x9cprimarily including a silver salt of an organic carboxylic acidxe2x80x9d may also refer to a substantially light insensitive organic silver salt that comprises only one silver salt of an organic carboxylic acid. The particles of substantially light-insensitive organic silver salts primarily containing a silver salt of an organic carboxylic acid preferably contain at least 50 mol % of the silver salt of an organic carboxylic acid.
The silver salt of an organic carboxylic acid primarily included in the particles of substantially light-insensitive organic silver salts according to the invention is preferably the silver salt of an aliphatic carboxylic acid, with silver salts of an aliphatic acid wherein the aliphatic carbon chain has at least 12 carbon atoms being particularly preferred e.g. silver laurate, silver stearate, silver hydroxystearate, silver behenate and silver arichidate. Especially preferred silver salts of an organic carboxylic acid primarily included in the particles of substantially light-insensitive organic silver salts according to the invention are silver behenate, silver palmitate and silver stearate
The substantially light-insensitive organic silver salts for use in the recording materials of the present invention together with the silver salts of an organic carboxylic acid are silver salts of any non-present aliphatic carboxylic acids known as fatty acids, wherein the aliphatic carbon chain has preferably at least 12 C-atoms, e.g. silver laurate, silver stearate, silver hydroxystearate, silver behenate and silver arichidate, silver salts of modified aliphatic carboxylic acids with thioether group as described e.g. in GB-P 1,111,492 and other organic silver salts as described in GB-P 1,439,478, e.g. silver benzoate, may likewise be used to produce a thermally developable silver image.
Non-halide ions containing phosphonium compounds have been found to promote the crystallinity of silver behenate. Preferred non-halide ion containing phosphonium compounds for promoting silver behenate crystallinity according to the present invention are:
PC01=(2-methoxyethyl)triphenylphosphonium toluenesulphonate
PC02=ethyltriphenylphosphonium toluenesulphonate
PC03=(2-triphenylphosphonium)ethyltriphenylphosphonium benzenesulphonate
Any known synthesis technique and any known dispersion technique can be used to produce the silver behenate of the present invention with the provision that the silver behenate in the thermographic recording material of the present invention fulfils the above criteria set forth under xe2x80x9csilver behenate characterizationxe2x80x9d.
In a preferred embodiment of the recording material of the present invention, the silver behenate is present in the thermosensitive element as particles of a substantially light-insensitive organic silver salts primarily comprising silver behenate. By the term xe2x80x9cprimarily comprising silver behenatexe2x80x9d it is meant that more silver behenate is present in the mixture of organic silver salts than other types of organic silver salts such as silver palmitate or silver stearate. The term xe2x80x9cprimarily comprising silver behenatexe2x80x9d may also refer to a substantially light insensitive organic silver salt that comprises only one organic silver salt, silver behenate.
A production process for particles of substantially light-insensitive organic silver salts, primarily comprising silver behenate, comprises: i) producing a solution or dispersion, A, comprising an alkali metal or ammonium salt of an organic compound with at least one acidic hydrogen atom and further comprising behenic acid in a mixture of water and an organic solvent at a temperature at which particles of substantially light insensitive organic silver salts including silver behenate do not undergo reduction; and ii) adding a quantity of an aqueous solution, B, of a silver salt containing an equal number of silver ions to the alkali or ammonium ions in the solution or dispersion, A.
The above described production process for particles of substantially light-insensitive organic silver salts primarily comprising silver behenate is further characterized in that the mixing number during the addition of the aqueous solution B to the solution or dispersion A is greater than or equal to 2xc3x9710xe2x88x924 during the production of the particles of substantially light-insensitive organic silver salts primarily comprising silver behenate. The mixing number is the ratio of the molar rate at which the silver salt is supplied to the solution A in a reactor to the molar rate at which the alkali or ammonium salt is circulated in the reactor. The mixing number can be expressed as follows:
mixing number,   MN  =            MN      0        xc3x97                  1        +                                            ⅆ                              V                B                                                    ⅆ              t                                xc3x97                      t                          V              0                                                  1        -                                            C              B                                      M              0                                xc3x97                                    ⅆ                              V                B                                                    ⅆ              t                                xc3x97          t                    
where       MN    0    =                              ⅆ                      V            B                                    ⅆ          t                    xc3x97              C        B                                      M          0                          V          0                    xc3x97              G        F            xc3x97              D        3            xc3x97      n                  ⅆ              V        B                    ⅆ      t        =      rate    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    addition    ⁢                  xe2x80x83            ⁢              xe2x80x83              ⁢    of    ⁢                  xe2x80x83            ⁢              xe2x80x83              ⁢    solution    ⁢          xe2x80x83        ⁢    B    ⁢          xe2x80x83        ⁢    in    ⁢          xe2x80x83        ⁢          dm      3        ⁢          /        ⁢    minute  
rate of addition of solution B in dm3/minute.
CB=concentration of solution B in moles/dm3 
M0=initial quantity of alkali metal or ammonium salt of an organic compound with at least one acidic hydrogen atom comprising behenic acid present in solution A in moles
V0=volume of solution A in the reactor in dm3 
GF=pumping number of the stirrer in the reactor
n=stirring rate in rotations/minute
D=stirrer diameter in dm
t=time in minutes
The pumping number of the stirrer in a reactor is the ratio of stirrer pumping rate to the product of the stirrer diameter cubed and the stirring rate (nxc3x97D3) and is dependent on a variety of factors, such as the ratio of stirrer to reactor diameter, the off-bottom clearance ratio of the stirrer and the Reynolds number of the stirrer, see for example A. Bakker and L. E. Gates, Chemical Engineering Progress pages 25 to 34 (December 1995) and Nagata, xe2x80x9cMixing Principles and Applicationsxe2x80x9d, J. Wiley and Sons, New York (1975), pages 136-139.
The mathematical nature of the above function for the mixing number is such that the mixing number will always increase as the preparation of the particles of organic silver salts primarily comprising silver behenate proceeds.
In a preferred production process of the present invention, the solution or dispersion A has a molar concentration of alkali metal or ammonium salt of organic carboxylic acid compound with at least one acidic hydrogen atom comprising behenic acid in the mixture of water and an organic solvent greater than 0.022.
In another preferred production process of the present invention, the organic solvent is present at between 20 and 80% by weight of the mixture and preferably between 35 and 65% by weight of the mixture. In a further preferred production process, the organic solvent is 2-butanone.
Surprisingly, it has been found from scanning electron micrographs of dispersions of particles of organic silver salt comprising silver behenate and from transition electron micrographs of thin slices of thermosensitive layers coated from dispersions of particles of organic silver salts primarily comprising silver behenate that the shape of these particles in general changes from rods to a more globular shape as the mixing number is increased. It has also been found that this change in particle morphology is accompanied by an increase in the crystallinity, as defined above, provided that the dispersion of these particles of organic silver salts primarily comprising silver behenate is carried out under similar conditions. In a preferred embodiment of the recording material of the present invention, at least 20% by weight of the particles of organic silver salts primarily comprising silver behenate are present as globular particles and, in a particularly preferred embodiment of the recording material of the present invention, at least 40% by weight of the particles of organic silver salts primarily comprising silver behenate are present as globular particles.
A production process for a dispersion of particles of substantially light-insensitive organic silver salt primarily including a silver salt of an organic carboxylic acid in a substantially solvent-free aqueous medium according to the present invention the process comprises: i) preparing an aqueous dispersion of one or more organic acids primarily including the organic carboxylic acid and a salt of an anionic surfactant such as alkylarylsulfonate; ii) substantially neutralizing the organic acids with aqueous alkali, thereby forming organic acid salts primarily including a salt of the organic carboxylic acid; (iii) adding an aqueous solution of a silver salt to completely convert the organic acid salts into their silver salts primarily including the silver salt of the organic carboxylic acid. Preferred organic carboxylic acids are aliphatic carboxylic acids, with aliphatic carboxylic acid wherein the aliphatic carbon chain has at least 12 carbon atoms being particularly preferred e.g. lauric acid, palmitic acid, stearic acid, hydroxystearic acid, behenic acid and arichidic acid. Especially preferred organic carboxylic acids are behenic acid, palmitic acid and stearic acid.
During the production process, the anionic surfactant is present in a molar ratio with respect to organic acid greater than 0.15 and the silver salt is added at a rate between 0.025 mol/(mol organic silver saltxc3x97min) and 2.25 mol/(mol organic silver saltxc3x97min).
In a preferred embodiment of the production process, the anionic surfactant is present in a molar ratio with respect to organic carboxylic acid greater than 0.25 and the silver salt is added at a rate between 0.03 mol/(mol organic silver saltxc3x97min) and 0.7 mol/(mol organic silver saltxc3x97min). In a further preferred embodiment, the molar ratio of anionic surfactant with respect to organic acid is greater than 0.3 and the rate of silver salt addition is between 0.04 mol/(mol organic silver saltxc3x97min) and 0.3 mol/(mol organic silver saltxc3x97min).
In another preferred embodiment, step (iii) of the production process of the present invention is carried out such that part the solution of organic carboxylic acid salts produced in step (ii) of the process is present in the reaction vessel prior to silver salt solution addition and part thereof is added simultaneously with the addition of the silver salt solution. In a particularly preferred embodiment, about 25 to 50% of the solution of acid salts produced in step (ii) is in the reaction vessel prior to silver salt addition. The remainder is added to the vessel after or during silver salt addition.
In another preferred embodiment the anionic surfactant is selected from the group consisting of: alkylsulfonate salts, alkarylsulfonate salts, aralkylsulfonate salts, arylsulfonate salts, alkylsulfate salts, aralkylsulfate salts, arylsulfate salts, alkarylsulfate salts and organic carboxylate salts. In a particularly preferred embodiment of the present invention the anionic surfactant is an alkarylsulfonate salt and in an especially preferred embodiment the anionic surfactant is an alkylbenzene sulfonate salt.
Suitable anionic surfactants for use the present invention include the commercially available:
Surfactant Nr. 1=MARLON(trademark) A-396, a sodium alkyl-phenylsulfonate from Hxc3xcls;
Surfactant Nr. 2=Marlon(trademark) A-S3, an alkylphenylsulfonic acid from Hxc3xcls neutralized with an alkali hydroxide;
Surfactant Nr. 3=ammonium 4-dodecylbenzene sulfonate;
Surfactant Nr. 4=ULTRAVON(trademark) W, a sodium arylsulfonate from Ciba-Geigy;
Surfactant Nr. 5=ERKANTOL(trademark) BX, a sodium diisopropyl-naphthalenesulfonate from BAYER;
Surfactant Nr. 6=ALKANOL(trademark) XC, a sodium nonylnaphthalene-sulfonate from DU PONT;
Surfactant Nr. 7=HOSTAPUR(trademark), a secondary alkanesulfonate from HOECHST;
Surfactant Nr. 8=MERSOLAT(trademark) H80, a sodium hexadecylsulfonate from Bayer;
Surfactant Nr. 9=HOSTAPAL(trademark) B, a sodium trisalkylphenyl-polyethyleneglycol(EO 7-8) sulphate from Hoechst;
Surfactant Nr. 10=TERGITOL(trademark) 4, a sodium 1-(2xe2x80x2-ethylbutyl)-4-ethylhexylsulphate from GOLDSCHMIDT;
In the above-described production process, the pH of the aqueous medium must be sufficiently low to avoid the oxidation of silver ions to silver oxide or silver hydroxide. A pH below 10 is usually sufficient. Additionally, the process temperature should be chosen so that it is above the melting point of the organic acid(s) used. In the case of an organic acid primarily comprising behenic acid a temperature of about 80 to 85xc2x0 C. is appropriate, in the case of an organic acid primarily comprising palmitic acid a temperature of 65xc2x0 C. is appropriate and in the case of stearic acid a temperature of around 75xc2x0 C. is preferred. The process should be carried out with stirring, the stirring rate being dependent upon the size of the stirrer relative to the size of the reaction vessel and the type of stirrer used. Stirring should be adjusted to avoid silver oxide or silver hydroxide formation due to insufficient mixing and also to avoid foaming. A stirring rate of between 200 and 1000 rpm, is preferably used. Also in a preferred embodiment a slight excess of an organic acid, for example behenic acid at an e.g. 2 mol % excess may be provided.
The size of the silver acid salt particles primarily containing the silver salt of a carboxylic acid can be varied by varying the rate of silver salt addition, the concentration of anionic surfactant and the temperature. The diameter of the particles generally increases with decreasing addition rate, decreasing anionic surfactant concentration, or increasing temperature.
In a further preferred embodiment the production process also includes a step in which the organic silver salts primarily including the silver salt of a carboxylic acid are subject to ultrafiltration. The ultrafiltration process removes ionic species and concentrates the dispersion of particles primarily containing the silver salt of a carboxylic acid by filtration through a cartridge-filter with a pore size sufficient to remove particles of organic silver salts with a molecular weight substantially smaller than the silver salt of a carboxylic acid. Cartridge-filters having a 10 000 to 500 000 MW cut-off have been found to be suitable for this purpose. In order to maintain the stability of the dispersion of particles primarily including the silver salt of a carboxylic acid during ultrafiltration it is necessary to maintain a minimum anionic surfactant concentration, but the counterion of the anionic surfactant can be changed, should the presence of the original counterion be undesirable in the thermographic recording material. For example, the sodium ions in Surfactant Nr 1 can be replaced by ammonium ions by washing with an ammonium nitrate solution during the ultrafiltration process to reduce the sodium ion concentration to below 100 ppm.
The above-mentioned process produces substantially light-insensitive organic silver salt particles primarily containing silver behenate in which the silver behenate has a crystallinity, as defined above for silver behenate, greater than 0.85 m2/g.
The above-mentioned process produces substantially light-insensitive organic silver salt particles primarily containing silver palmitate in which the silver palmitate has a crystallinity, as defined above for silver palmitate, greater than 3.09 m/g0.5.
The above-mentioned process produces substantially light-insensitive organic silver salt particles primarily containing silver stearate in which the silver stearate has a crystallinity, as defined above for silver stearate, greater than 2.2 m/g5.
In the case of dried particles of organic silver salts primarily comprising silver behenate, silver palmitate, or silver stearate with higher crystallinity, it has been found that recording materials, according to the present invention can be produced if dispersions thereof are produced using dispersion techniques in which the particles themselves are subjected to as little damage as possible commensurate with achieving a satisfactory dispersion quality. In a preferred embodiment, microfluidizers, ultrasonic apparatuses, rotor stator mixers etc. may be used for dispersion.
Surfactants and dispersants aid the dispersion of ingredients or reactants which are insoluble in a particular dispersion medium. The thermographic recording materials of the present invention may contain one or more such surfactants, which may be anionic, non-ionic or cationic, and one or more such dispersants.
Suitable dispersants are natural polymeric substances, synthetic polymeric substances and finely divided powders, for example finely divided non-metallic inorganic powders such as silica. Suitable hydrophilic natural or synthetic polymeric substances contain one or more hydroxyl, carboxyl or phosphate groups, e.g. protein-type binders such as gelatin, casein, collagen, albumin and modified gelatin; modified cellulose; starch; modified starch; modified sugars; modified dextrans etc. Examples of suitable hydrophilic synthetic polymeric substances are polyvinylalcohol; polyvinylpyrrolidone; polyacrylic acid; and polymethacrylic acid and their copolymers.
Suitable organic reducing agents for the reduction of particles of organic silver salts primarily comprising silver behenate, silver palmitate, or silver stearate are organic compounds containing at least one active hydrogen atom linked to O, N, or C. Suitable reducing agents include: aromatic di- and tri-hydroxy compounds; aminophenols; METOL (tradename); p-phenylene-diamines; alkoxynaphthols, e.g. 4-methoxy-1-naphthol described in U.S. Pat. No. 3,094,41; pyrazolidin-3-one type reducing agents, e.g. PHENIDONE (tradename); pyrazolin-5-ones; indan-1,3-dione derivatives; hydroxytetrone acids; hydroxytetronimides; hydroxylamine derivatives such as those described in U.S. Pat. No. 4,082,901; hydrazine derivatives; and reductones such as, ascorbic acid. See also U.S. Pat. No. 3,074,809, U.S. Pat. No. 3,080,254, U.S. Pat. No. 3,094,417 and U.S. Pat. No. 3,887,378 for further examples.
Useful aromatic di- and tri-hydroxy compounds include those having at least two hydroxy groups in ortho- or para-position on the same aromatic nucleus, such as benzene nucleus, hydroquinone and substituted hydroquinones, catechol, pyrogallol, gallic acid and gallic acid esters. Polyhydroxy spiro-bis-indane compounds are particularly preferred.
Among the catechol-type reducing agents, i.e. reducing agents containing at least one benzene nucleus with two hydroxy groups(xe2x80x94OH) in ortho-position, the following are preferred: catechol, 3-(3xe2x80x2,4xe2x80x2-dihydroxyphenyl) propionic acid, 1,2-dihydroxybenzoic acid, gallic acid and esters e.g. methyl gallate, ethyl gallate, propyl gallate, tannic acid, and 3,4-dihydroxy-benzoic acid esters. Particularly preferred catechol-type reducing agents are described in EP-B 692 733, and EP-A 903 625.
Other suitable reducing agents, particularly for photothermographic recording materials, are sterically hindered phenols, bisphenols and sulfonamidophenols.
Combinations of reducing agents may also be used that on heating the agents become reactive partners in the reduction of the substantially light-insensitive organic silver salts primarily comprising silver behenate, silver palmitate, or silver stearate. For example, combinations of reducing agents with sulfonamidophenols are described in the periodical Research Disclosure, February 1979, item 17842, in U.S. Pat. No. 4,36xc2x0,581 and U.S. Pat. No. 4,782,004, and in EP-A 423 891. Combinations of sterically hindered phenols with sulfonyl hydrazide reducing agents are disclosed in U.S. Pat. No. 5,464,738. Combinations of trityl hydrazides and formyl-phenyl-hydrazides are disclosed in U.S. Pat. No. 5,496,695. Combinations of trityl hydrazides and formyl-phenyl-hydrazides with diverse auxiliary reducing agents are disclosed in U.S. Pat. No. 5,545,505, U.S. Pat. No. 5.545.507 and U.S. Pat. No. 5,558,983. Acrylonitrile compounds are disclosed in U.S. Pat. No. 5,545,515 and U.S. Pat. No. 5,635,339. 2-substituted malondialdehyde compounds are disclosed in U.S. Pat. No. 5,654,130. Organic reducing metal salts, such as stannous stearate, have also been used in such reducing agent combinations, as disclosed in U.S. Pat. No. 3,460,946 and U.S. Pat. No. 3,547,648. Sterically hindered phenols and bisphenols have also been used in such reducing agent combinations such as those described in U.S. Pat. No. 4,001,026 and U.S. Pat. No. 3,547,648, respectively.
The silver image density depends on the coverage of the above defined reducing agent(s) and organic silver salt(s) and has preferably be adjusted so that, upon heating above 100xc2x0 C., an optical density of at least 2.5 may be obtained. Preferably, at least 0.10 moles of reducing agent per mole of organic silver salts is used.
The thermosensitive element of the recording material of the present invention may additionally comprise at least one polycarboxylic acid and/or anhydride thereof in a molar percentage of at least 20 with respect to all the organic silver salt(s) present and in thermal working relationship therewith. The polycarboxylic acid may be aliphatic (saturated as well as unsaturated aliphatic and also cycloaliphatic) or an aromatic polycarboxylic acid. These polycarboxylic acids may be substituted with for example, an alkyl, hydroxyl, nitro or halogen group. They may be used in anhydride form or partially esterified so long as at least two free carboxylic acids remain or are available during the heat recording step.
In a preferred embodiment, the polycarboxylic acids are saturated aliphatic dicarboxylic acids containing at least 4 carbon atoms, such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, nonane-dicarboxylic acid, decane-dicarboxylic acid, and undecane-dicarboxylic acid.
In another preferred embodiment, the polycarboxylic acids are unsaturated dicarboxylic acids, such as maleic acid, citraconic acid, itaconic acid and aconitic acid. Suitable polycarboxylic acids are citric acid and derivatives thereof, acetonedicarboxylic acid, iso-citric acid, or xcex1-ketoglutaric acid.
In yet another preferred embodiment, the polycarboxylic acids are aromatic polycarboxylic acids, such as ortho-phthalic acid, 3-nitro-phthalic acid, tetrachlorophthalic acid, mellitic acid, pyromellitic acid, or trimellitic acid and the anhydrides thereof.
The film-forming binder of the thermosensitive element containing organic silver salts primarily comprising silver behenate, silver palmitate, or silver stearate may be any kind of natural, modified natural or synthetic resin or mixtures of such resins in which the organic silver salts may be dispersed homogeneously in either aqueous or solvent media. In a preferred embodiment, the binder may be cellulose derivatives such as ethylcellulose, cellulose esters, e.g. cellulose nitrate, carboxymethylcellulose, starch ethers, galactomannan, polymers derived from xcex1,xcex2-ethylenically unsaturated compounds such as polyvinyl chloride, after-chlorinated polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, copolymers of vinyl chloride and vinyl acetate, polyvinyl acetate and partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl acetals that are made from polyvinyl alcohol as starting material in which only a part of the repeating vinyl alcohol units may have reacted with an aldehyde, preferably polyvinyl butyral, copolymers of acrylonitrile and acrylamide, polyacrylic acid esters, polymethacrylic acid esters, polystyrene, polyethylene, or mixtures of any of the above.
A particularly suitable polyvinyl butyral containing a minor amount of vinyl alcohol units is marketed under the trade name BUTVAR(trademark) B79 of Monsanto USA and provides a good adhesion to paper and properly subbed polyester supports.
The layer containing the organic silver salt is commonly coated onto a support in sheet or web form from an organic solvent containing the binder dissolved therein. It may also be applied from an aqueous medium containing a water-dispersible binder.
Suitable water-soluble, film-forming binders for use in thermographic and photothermographic recording materials according to the present invention include polyvinyl alcohol, polyacrylamide, polymethacrylamide, polyacrylic acid, polymethacrylic acid, polyvinylpyrrolidone, polyethyleneglycol, proteinaceous binders such as gelatin, modified gelatins such as phthaloyl gelatin, polysaccharides, such as starch, gum arabic, and dextran, and water-soluble cellulose derivatives. A preferred water-soluble binder for use in the thermographic and photothermographic recording materials of the present invention is gelatin.
Suitable water-dispersible binders for use in the thermographic and photothermographic recording materials of the present invention include any water-insoluble polymers such as: water-insoluble cellulose derivatives, polyurethanes, polyesters, polycarbonates and polymers derived from xcex1,xcex2-ethylenically unsaturated compounds such as after-chlorinated polyvinyl chloride, partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl acetals preferably polyvinyl butyral, and homopolymers and copolymers produced using monomers selected from the group consisting of: vinyl chloride, vinylidene chloride, acrylonitrile, acrylamides, methacrylamides. methacrylates, acrylates, methacrylic acid, acrylic acid, vinyl esters, styrenes, dienes and alkenes, or mixtures of any of the above. It should be noted that there is no clear transition between a polymer dispersion and a polymer solution in the case of very small polymer particles because the smallest particles of the polymer are dissolved while those slightly larger remain in dispersion.
Preferred water-dispersible binders for use according to the present invention are water-dispersible film-forming polymers with covalently bonded ionic groups selected from the group consisting of: sulfonate, sulfinate, carboxylate, phosphate, quaternary ammonium, tertiary sulfonium, and quaternary phosphonium groups. Further preferred water-dispersible binders for use according the present invention are water-dispersible film-forming polymers with covalently bonded moieties with one or more acid groups.
Water-dispersible binders with crosslinkable groups, such as epoxy groups, aceto-acetoxy groups and crosslinkable double bonds are also preferred.
Other preferred water-dispersible binders for use in the thermographic and photothermographic recording materials of the present invention are polymer latexes. Preferably the dispersible polymer for use as a latex has some hydrophilic functionality. Polymers with hydrophilic functionality for forming an aqueous polymer dispersion (latex) are described in U.S. Pat. No. 5,006,451. However, in that patent the polymers serve to form a barrier layer preventing unwanted diffusion of vanadium pentoxide present as an antistatic agent. This use is unrelated to the use contemplated in the present invention.
The weight ratio of binder to organic silver salts ratio is preferably in the range of 0.2 to 6, and the thickness of the recording layer is preferably in the range of 5 to 50 xcexcm.
The above mentioned binders or mixtures thereof may be used in conjunction with waxes or xe2x80x9cheat solventsxe2x80x9d also called xe2x80x9cthermal solventsxe2x80x9d or xe2x80x9cthermosolventsxe2x80x9d to improve the reaction speed of the redox-reaction at elevated temperatures. The term xe2x80x9cheat solventxe2x80x9d in this application refers to a non-hydrolyzable organic material which is in a solid state in the recording layer at temperatures below 50xc2x0 C. but becomes a plasticizer for the recording layer in the heated region and/or liquid solvent for at least one of the redox-reactants, such as the reducing agent for the organic silver salts, at temperatures above 60xc2x0 C.
In order to obtain a neutral black image tone in the higher densities and neutral grey in the lower densities, the thermosensitive element of the present invention preferably contains a toning agent known from thermography or photothermography in thermal working relationship with the organic silver salts and reducing agents.
Suitable toning agents include the phthalimides and phthalazinones within the scope of the general formulae described in U.S. Pat. No. 4,082,901. Other useful toning agents are disclosed in U.S. Pat. No. 3,074,809, U.S. Pat. No. 3,446,648 and U.S. Pat. No. 3,844,797. Particularly useful toning agents are the heterocyclic toner compounds of the benzoxazine dione or naphthoxazine dione type as disclosed in GB 1,439,478, U.S. Pat. No. 3,951,660 and U.S. Pat. No. 5,599,647. A toner compound particularly suited for use in combination with polyhydroxy benzene reducing agents is 3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine, described in U.S. Pat. No. 3,951,660.
In order to obtain improved shelf-life and reduced fogging, stabilizers and antifoggants may be incorporated into the recording materials of the present invention. Examples of suitable stabilizers and antifoggants and their precursors, which can be used alone or in combination, include the thiazolium salts described in U.S. Pat. No. 2,131,038 and 2,694,716; the azaindenes described in U.S. Pat. No. 2,886,437 and U.S. Pat. No. 2,444,605; the urazoles described in U.S. Pat. No. 3,287,135; the sulfocatechols described in U.S. Pat. No. 3,235,652; the oximes described in GB 623,448; the thiuronium salts described in U.S. Pat. No. 3,220,839; the palladium, platinum and gold salts described in U.S. Pat. No. 2,566,263 and U.S. Pat. No. 2,597,915; the tetrazolyl-thio-compounds described in U.S. Pat. No. 3,700,457; the mesoionic 1,2,4-triazolium-3-thiolate stabilizer precursors described in U.S. Pat. No. 4,404,390 and U.S. Pat. No. 4,351,896; the tribromomethyl ketone compounds described in EP-A 600 587; the combination of isocyanate and halogenated compounds described in EP-A 600 586; the vinyl sulfone and xcex2-halo sulfone compounds described in EP-A 600 589; and those compounds mentioned in this context in Chapter 9 of xe2x80x9cImaging Processes and Materials, Neblette""s 8th editionxe2x80x9d, by D. Klosterboer, edited by J. Sturge, V. Walworth and A. Shepp, page 279, Van Nostrand (1989); in Research Disclosure 17029 published in June 1978; and in the references cited in all these documents.
The recording material of the present invention may contain, in addition to the ingredients mentioned above, other additives such as free fatty acids, surface-active agents, antistatic agents, such as non-ionic antistatic agents including a fluorocarbon group as in F3C(CF2)6CONH(CH2CH2O)xe2x80x94H, silicone oil, such as BAYSILONE(trademark) xc3x96l A (BAYER AG, GERMANY), ultraviolet light absorbing compounds, pigments reflecting white light and/or ultraviolet radiation, and optical brightening agents.
The support for the thermosensitive element according to the present invention may be transparent, translucent or opaque. For instance, it may have a white light reflecting aspect. The support is preferably a thin flexible carrier such as paper, polyethylene coated paper, or a transparent resin film, such as films made of a cellulose ester, such as cellulose triacetate, polypropylene, polycarbonate or a polyester, such as polyethylene terephthalate. In a preferred embodiment, a paper base substrate is present and may contain white light reflecting pigments: such pigments may optionally also be applied in an interlayer between the recording material and the paper base substrate.
The support may be in sheet, ribbon, or web form and may be subbed if necessary to improve adherence to the thermosensitive element. The support may be made of an opacified resin composition, such as polyethylene terephthalate opacified by means of pigments optionally in combination with micro-voids and may be optionally coated with an opaque pigment-binder layer. Such a support may be called synthetic paper or paperlike film. Information about such supports can be found in EP 194 106, EP 234 563, U.S. Pat. No. 3,944,699, U.S. Pat. No. 4,187,113, U.S. Pat. No. 4,780,402, and U.S. Pat. No. 5,059,579. Should a transparent base be used, the base may be colorless or colored, e.g. having a blue color.
One or more backing layers may be provided to control physical properties such as curl and static.
The outermost layer of the recording material may, in different embodiments of the present invention, be the outermost layer of the thermosensitive element, a protective layer applied to the thermosensitive element or a layer on the opposite side of the support to the thermosensitive element.
According to a preferred embodiment of the recording material, of the present invention, the thermosensitive element is provided with a protective layer to avoid local deformation of the thermosensitive element and to improve resistance against abrasion.
The protective layer preferably comprises a binder, which may be solvent-soluble, solvent-dispersible, water-soluble or water-dispersible. Among the solvent-soluble binders polycarbonates, as described in EP-A 614 769, are particularly preferred. However, water-soluble or water-dispersible binders are preferred for the protective layer, as coating can be performed from an aqueous composition and mixing of the protective layer with the immediate underlayer can be avoided by using a solvent-soluble or solvent-dispersible binder in the immediate underlayer.
A protective layer according to the present invention may comprise in addition a thermomeltable particle optionally with a lubricant present on top of the protective layer as described in WO 94/11199. In a preferred embodiment at least one solid lubricant having a melting point below 150xc2x0 C. and at least one liquid lubricant in a binder is present, wherein at least one of the lubricants is a phosphoric acid derivative.
In an embodiment of the present invention, the outermost layer of the recording material may comprise a water-soluble binder, a water-dispersible binder, or a mixture of a water-soluble and a water-dispersible binder. Suitable water-soluble binders for the outermost layer include gelatin, polyvinylalcohol, cellulose derivatives or other polysaccharides, hydroxyethyl-cellulose, and hydroxypropylcellulose, etc. Hardenable binders are preferred, with polyvinylalcohol being particularly preferred. Suitable water-dispersible binders include polymeric latexes.
The outermost layer of the present invention may be crosslinked. Crosslinking can be achieved using crosslinking agents such as those described in WO 95/12495 for protective layers, including tetra-alkoxysilanes, polyisocyanates, zirconates, titanates, melamine resins etc., Tetraalkoxysilanes, such as tetramethylorthosilicate and tetraethylorthosilicate, are preferred.
The outermost layer of the recording material of the present invention may comprise a matting agent. Suitable matting agents are described in WO 94/11198 and include talc particles. Matting agents optionally protrude from the outermost layer.
Solid or liquid lubricants or combinations thereof are suitable for improving the slip characteristics of the recording materials of the present invention.
Solid lubricants may include: polyolefin waxes, ester waxes, polyolefin-polyether block copolymers, amide waxes, polyglycols, fatty acids, fatty alcohols, natural waxes and solid phosphoric acid derivatives. Preferred solid lubricants are thermomeltable particles such as those described in WO 94/11199.
Liquid lubricants may include: fatty acid esters such as glycerine trioleate, sorbitan monooleate and sorbitan trioleate, silicone oil derivatives and phosphoric acid derivatives.
Silver halide is a preferred photosensitive species of the present invention. It is capable, upon exposure to light, of forming species capable of catalyzing reduction of the organic silver salt primarily comprising silver behenate, silver palmitate, or silver stearate of the present invention.
The photosensitive silver halide used in the present invention may be employed in a range of 0.1 to 100 mol percent of substantially light-insensitive silver salts. In a preferred embodiment, the silver halide is present in a range from 0.2 to 80 mol percent. In a more preferred embodiment it is present in a range from 0.3 to 50 mol percent. In a further more preferred embodiment, it is present in a range from 0.5 to 35 mol percent. Finally in a still further preferred embodiment, the silver halide is present in a range from 1 to 12 mol percent of substantially light-insensitive organic silver salts.
The silver halide may be any photosensitive silver halide such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, etc. The silver halide may be in any form which is photosensitive including, but not limited to, cubic, orthorhombic, tabular, tetrahedral, octagonal, etc. and may have epitaxial growth of crystals thereon.
The silver halide used in the present invention may be employed without modification. However, it may be chemically sensitized with a chemical sensitizing agent such as a compound containing sulphur, selenium, tellurium etc., or a compound containing gold, platinum, palladium, iron, ruthenium, rhodium, or iridium, etc., a reducing agent such as a tin halide, etc., or a combination thereof. The details of these procedures are described in T. H. James, xe2x80x9cThe Theory of the Photographic Processxe2x80x9d, Fourth Edition, Macmillan Publishing Co. Inc., New York (1977), Chapter 5, pages 149 to 169.
The thermosensitive element, according to the present invention, may contain an infra-red sensitizer, an ultra-violet light sensitizer or a visible light sensitizer. Suitable sensitizers include cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol and xanthene dyes. Useful cyanine dyes include those having a basic nucleus, for example a thiazoline nucleus, an oxazoline nucleus, a pyrroline nucleus, a pyridine nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus and an imidazole nucleus. Preferred merocyanine dyes include those having not only the above described basic nuclei but also acid nuclei, for example a thiohydantoin nucleus, a rhodanine nucleus, an oxazolidinedione nucleus, a thiazolidinedione nucleus, a barbituric acid nucleus, a thiazolinone nucleus, a malononitrile nucleus and a pyrazolone nucleus. Of the above described cyanine and merocyanine dyes, those having imino groups or carboxyl groups are particularly preferred.
Suitable infra-red sensitizers include those disclosed in EP-A 465 078, EP-A 559 101, EP-A 616 014, EP-A 635 756, JN 03-080251, JN 03-163440, JN 05-019432, JN 05-072662, JN 06-003763, U.S. Pat. No. 4,515,888, U.S. Pat. No. 4,639,414, U.S. Pat. No. 4,713,316, U.S. Pat. No. 5,258,282 and U.S. Pat. No. 5,441,866.
According to the present invention the thermosensitive element may further include a supersensitizer. Preferred supersensitzers are compounds selected from the group consisting of: mercapto-compounds, disulfide-compounds, stilbene compounds, organoborate compounds and styryl compounds. Suitable supersensitizers for use with infra-red spectral sensitizers are disclosed in EP-A 559 228, EP-A 587 338, U.S. Pat. No. 3,877,943, U.S. Pat. No. 4,873,184 and EP-A 821 271.
In addition to other ingredients, the recording materials used in the present invention may also contain antihalation or acutance dyes which absorb light which has passed through the photosensitive thermally developable photographic material, thereby preventing its reflection. Such dyes may be incorporated into the photosensitive thermally developable photographic material or in any other layer of the photographic material of the present invention.
In a preferred embodiment of the recording material of the present invention an antistatic layer is applied to the outermost layer not comprising at least one solid lubricant having a melting point below 150xc2x0 C. and at least one liquid lubricant in a binder, wherein at least one of the lubricants is a phosphoric acid derivative.
The coating of any layer of the recording material of the present invention may proceed by any coating technique, e.g. such as described in Modern Coating and Drying Technology, edited by Edward D. Cohen and Edgar B. Gutoff, (1992) VCH Publishers Inc., New York, U.S.A.
Thermographic imaging is carried out by the image-wise application of heat either in analogue fashion by direct exposure through an image of by reflection from an image, or in digital fashion pixel by pixel either by using an infra-red heat source, for example with a Nd-YAG laser or other infra-red laser, with a thermographic material preferably containing an infra-red absorbing compound, or by direct thermal imaging with a thermal head.
In thermal printing, image signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal printhead consists of microscopic heat resistor elements, which convert the electrical energy into heat via Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermal paper wherein the chemical reaction resulting in color development takes place. Such thermal printing heads may be used in contact or close proximity with the recording material. The operating temperature of common thermal printheads is in the range of 300 to 400xc2x0 C. and the heating time per picture element (pixel) may be less than 1.0 ms, the pressure contact of the thermal printhead with the recording material being, e.g., 200-500 g/cm2 to ensure a good transfer of heat.
In order to avoid direct contact of the thermal printing heads with the outermost layer on the same side of the support as the thermosensitive element when this outermost layer is not a protective layer, the image-wise heating of the recording material with the thermal printing heads may proceed through a contacting but removable resin sheet which prevents transfer of recording material during heating.
The image signals for modulating the laser beam or current in the micro-resistors of a thermal printhead are obtained directly, for example, from opto-electronic scanning devices. They may also be obtained from an intermediary storage means such as a magnetic disc or tape or optical disc storage medium. Optionally, these intermediary means may be linked to a digital image work station where the image information can be processed to satisfy particular needs.
Activation of the heating elements can be power-modulated or pulse-length modulated at constant power. The image-wise heating can be carried out such that heating elements not required to produce an image pixel generate an amount of heat (He) in accordance with the following formula:
0.5 HD less than He less than HD
wherein HD represents the minimum amount of heat required to cause visible image formation in the recording material.
EP-A 654 355 discloses a method for making an image by image-wise heating with a thermal head having energizable heating elements. The activation of the heating elements is executed duty cycled pulsewise. When used in thermographic recording operating with thermal printheads, the thermographic recording materials are not suitable for reproducing images with fairly large number of grey levels as is required for continuous tone reproduction. EP-A 622 217 discloses a method for making an image using a direct thermal imaging element producing improvements in continuous tone reproduction.
Image-wise heating of the recording material can also be carried out using an electrically resistive ribbon incorporated into the material. Image- or pattern-wise heating of the recording material may also proceed by means of pixel-wise modulated ultra-sound, using, for example an ultrasonic pixel printer as described e.g. in U.S. Pat. No. 4,908,631.
Photothermographic recording materials, according to the present invention, may be exposed with radiation of wavelength between an X-ray wavelength and a 5 microns wavelength. The image may be obtained by pixel-wise exposure with: a finely focused light source, such as a CRT light source; a UV, visible or IR wavelength laser, such as a He/Ne-laser or an IR-laser diode, e.g. emitting at 780 nm, 830 nm or 850 nm; a light emitting diode, for example one emitting at 659 nm; or by direct exposure to the object itself or an image therefrom with appropriate illumination e.g. with UV, visible or IR light.
For the thermal development of image-wise exposed photothermographic recording materials, according to the present invention, any sort of heat source can be used that enables the recording materials to be uniformly heated to the development temperature in a time acceptable for the application concerned. Contact heating, radiative heating, microwave heating, etc. may be used.
Direct thermal imaging and photothermographic imaging can be used for the production of transparencies and for reflection type prints. Application of the present invention is envisaged in the fields of both graphics images requiring high contrast images with a very steep dependence of print density upon applied dot energy and of continuous tone images requiring a weaker dependence of print density upon applied dot energy, such as required in the medical diagnostic field. In the hard copy field, recording materials on a white opaque base are used, whereas in the medical diagnostic field black-imaged transparencies are widely used in inspection techniques operating with a light box.
The invention is illustrated hereinafter by way of invention examples and comparative examples.
It will be understood by one skilled in the art that examples given only for organic silver salts primarily comprising silver behenate, silver palmitate or silver stearate might be adapted, in light of this disclosure, for application to the other two carboxylic acids of the group or other carboxylic acids in general. The same is true for techniques and materials described above which may be omitted in the examples. Commercially available forms of behenic, palmitic, and stearic acids may be used, each of which tend to contain at least trace amounts of all three of these acids, as well as other carboxylic acids. Therefore, this disclosure also teaches one skilled in the art to obtain particles of any carboxylic acids commonly found in commercially available behenic, palmitic, or stearic acids. Specially prepared and relatively pure forms of the these and other carboxylic acids may also be employed in the disclosed process.
The percentages and ratios given in these examples are by weight unless otherwise indicated. The ingredients used in the invention and comparative examples, other than those mentioned above, are:
as organic silver salt:
AgB=silver behenate;
Ag Pa=silver palmitate;
AgSt=silver stearate;
as binders:
PVB=BUTVAR(trademark) B79, a polyvinyl butyral from Monsanto;
K7598=type K7598, a calcium-free gelatin from AGFA-GEVAERT GELATINEFABRIEK vorm, KOEPFF and Sxc3x96HNE;
K17881=type K17881, a calcium-free gelatin from AGFA-GEVAERT GELATINEFABRIEK vorm. KOEPFF and Sxc3x96HNE;
K16353=type K16353, a calcium-free high viscosity gelatin from AGFA-GEVAERT GELATINEFABRIEK vorm. KOEPFF and Sxc3x96HNE;
LATEX 01=a latex of a copolymer of 50% by weight of butadiene and 50% by weight of methyl methacrylate;
LATEX 02=a latex of a terpolymer of 47.5% by weight of butadiene, 47.5% by weight of methyl methacrylate and 5% by weight of itaconic acid;
LATEX 03=a 24% by weight aqueous latex of a polymer produced by copolymerizing a monomer mixture comprising 42% by weight of n-butyl acrylate, 53% by weight of styrene, 2% by weight of itaconic acid and 3% by weight of CH2xe2x95x90C(CH3)CONHxe2x80x94(CH2)10xe2x80x94CONHC6H4-pxe2x80x94SO3K followed by desalting and adjusting to pH 5.4 with ammonia;
as reducing agents:
R01=ethyl 3,4-dihydroxybenzoate;
R02=3(3xe2x80x2,4xe2x80x2-dihydroxyphenyl)propionic acid;
R03=LOWINOX(trademark) 22IB46, 2-propyl-bis(2-hydroxy-3,5-dimethylphenyl) from CHEM. WERKE LOWI;
as toning agents:
TA01=benzo[e][1,3]oxazine-2,4-dione;
TA02=7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione (see formula I below) 
TA03=succinimide;
TA04=phthalazinone;
TA05=phthalazine;
as levelling agent:
oil=Baysilone(trademark), a silicone oil from Bayer AG;
as stabilizers:
S01=tetrachlorophthalic anhydride;
S02=adipic acid;
S03=benzotriazole.